Climate change effects on hydropower in California

[ The main impact is on hydropower, the largest source of renewable electric power in California, and besides natural gas, the only other way to balance wind and solar power, (unless utility-scale energy storage batteries become available).  But hydropower is often unavailable (i.e. drought, low reservoirs, to provide months of agriculture and drinking water, protect fisheries, etc).

Alice Friedemann   www.energyskeptic.com  author of “When Trucks Stop Running: Energy and the Future of Transportation”, 2015, Springer and “Crunch! Whole Grain Artisan Chips and Crackers”]

CEC. September 2014. Climate change impacts on generation of wind, solar, and hydropower in California. California Energy Commission Lawrence Livermore National Laboratory CEC-500-2014-111

Excerpts:

The study findings for hydroelectric power generation show significant reductions that are a consequence of the large predicted reduction in annual mean precipitation in the global climate models used. Reduced precipitation and resulting reductions in runoff result in reduced hydropower generation in all months and elevation bands. These results indicate that a future that is both drier and warmer would have important impacts on the ability to generate electricity from hydropower.

Increased production of electricity from renewables, although desirable from environmental and other viewpoints, may create difficulties in consistently meeting demand for electricity and may complicate the job of operating the state’s transmission system. This would be true of any major change in electrical supply portfolio but is especially so when the proportion of weather-dependent renewables- which are subject to uncontrolled fluctuations-is increased.

Climate change may affect the ability to generate needed amounts of electricity from weather-dependent renewable resources. This could compromise California’s ability to meet renewable targets. For example, it is well documented that climate change is affecting the seasonal timing of river flows such that less hydropower is generated during months of peak demand and maximum electricity value. Generating solar and wind power may also be impacted by long-term changes in climate.

A changing climate may bring increases or decreases in mean wind speeds, as well as greater or lesser variation wind speeds. These changes could make long- term planning for wind energy purposes problematic. Some regions where continued wind development is occurring, such as California and the Great Plains, may be especially susceptible to climate change because the wind regimes of these regions are dominated by one particular atmospheric circulation pattern.

CHAPTER 4: Hydropower

Numerous modeling studies, starting with Gleick (1987), have predicted that anthropogenic climate change will have significant impacts on the natural hydrology of California, with implications for water scarcity, flood risk, and hydropower generation. The best-known of these impacts are straightforward consequences of increased temperatures: a reduced fraction of precipitation falling as snow, reduced snow extent and snow-water equivalent, and earlier melting of snow. An increased fraction of precipitation as rain in turn result s in increased wintertime runoff and river flow; earlier and reduced snow melt results in reduced late-season runoff and river flow. Despite the well-known lack of consensus among global climate models about future changes in annual California precipitation amounts, the effects just mentioned are robustly predicted because they result from warming, about which there is consensus. Confidence in these predictions is increased by observational studies that show these changes to be underway as well as by studies involving both observations and modeling that indicate that observed changes in western U.S. hydrology are too rapid to be explained entirely by natural causes.

One possible consequence of human-caused changes in mountain hydrology in the western United States is changes in hydropower production, especially from high-altitude facilities on watersheds that have historically been snow-dominated. This concern is especially acute, since a majority of the state’s hydropower is produced in facilities of this type. Furthermore, these high-elevation facilities have relatively little storage capacity, implying limited capability to adapt to changes in climate.

A shift toward earlier-in-the-year snowmelt and runoff would tend to produce similar changes in the timing of hydropower generation. In particular, in the absence of adequate storage capacity, it might become difficult to produce power at the end of the dry season, when demand for electricity can be very high.

On the other hand, a large enough reservoir could store enough water to effectively buffer this problem and allow power generation throughout the dry season. This means that the effects of climate change on hydropower generation will depend strongly on reservoir size. And of course on altitude, being greatest at intermediate altitudes where slight warming will raise the temperature above freezing. Watersheds that are already rain-dominated, or are well below freezing, will not exhibit the effects discussed here in the near future.

Of course, besides issues of seasonal timing, a significant increase or decrease in annual total precipitation would be an important benefit or detriment (respectively) to hydropower generation.

The published literature largely supports this picture.

Madani and Lund (2009) looked at hydropower generation in 137 high-elevation systems under three simple climate change scenarios: wet, dry, and warming only. It found that existing storage capacity is sufficient to largely compensate for expected changes in the seasonal timing of snowmelt, runoff, and river flow. A hypothetical decrease in annual total runoff, however, translates more directly into a corresponding reduction in energy generation. The predicted response to a hypothetical increase in annual runoff, however, is not symmetrical: this scenario results in increased spill and very little increase in energy production.

Results

The research team’s results for optimized energy generation are driven primarily by large projected reductions in precipitation in the future climate scenario. In the study area, annual mean precipitation in the future period is reduced by as much as 30 percent compared to in the historical reference period. Because of the complex relationships among precipitation, evapotranspiration, and runoff, these already- large precipitation decreases produce proportionately larger reductions in run off and stream flow. In other words, the percentage reductions in runoff and river flow exceed those in precipitation.

This phenomenon is exaggerated by the tendency for warming to result in increased evaporation.

Disproportionate decreases in runoff in a dry future- climate scenario are seen in other modeling studies Jones et al. (2005) investigated changes in runoff in several surface hydrology models in response to a hypothetical 1% change in precipitation and found responses ranging from 1.8 to 4.1%; that is, the percentage response in runoff was anywhere from roughly double to roughly 4x the percentage change in precipitation.

Reduced precipitation and resulting reductions in runoff result in reduced hydropower generation in all months and elevation bands

These results indicate that a future that is both drier and warmer would have important impacts on the ability to generate electricity from hydropower.

 

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